When more than one type or color of liquid ink is used to print a color image, various print quality issues must be addressed relating to how the different types of ink interact on the print medium. This is particularly the case in ink jet printers in which different types of liquid ink from different "pens" (ink cartridges, nozzles, or other ink delivery devices) are sequentially applied onto the same location or contiguous locations of the print medium during the same traverse of the carriage across the print medium.
Two commonly encountered image defects are "Halo" and "Bleed". A stylized representation of "Halo" is depicted in exaggerated form in FIG. 1, and typically occurs at the boundaries between adjacent colored areas producing a white gleam (H) at the interfaces (I). When Halo occurs, it is believed that the ink shrinks away from the boundary of each region dries, leaving an exposed line of bare print medium in the boundary region.
A stylized representation of "Bleed" is depicted in exaggerated form in FIG. 2, and typically occurs on the edges of lines or boxes, where one color of ink is applied next to another color of ink. When Bleed occurs, it is believed that there is a migration of colorant into adjacent regions causing rough or expanded boundaries (B).
Both problems of Bleed and Halo can be mitigated by increasing the time delay of the deposition between the inks. However, it is not always practical to wait for one type of ink to dry completely before applying another. Not only would such an expedient result in a much longer printing process, it actually can produce other unwanted image artifacts. For example, in a typical ink jet printer, the ink is applied in horizontal swaths; if the ink in one swath is completely dry before the next swath is applied, there will be a noticeable line at the boundary between the two swaths. There are a number of other image quality defects that can be impacted by changes in time-delay such as mottle, gloss uniformity. Moreover, a certain amount of mixing of two different colors is desirable when printing a large area of a secondary color made from more than one color.
By introducing modifications to the colorant and/or by adding or modifying surfactant, penetrant, inorganic ion, polymeric and/or other chemical components of the ink formulation, physical properties (for example, viscosity, surface tension, and boiling point) and chemical properties (for example, film forming ability and binding ability) can be altered to change the chemical interactions within the same type of ink, between the different inks, and between each ink and the underlying media, thereby changing the manner a particular type of ink spreads and interacts with other inks and with the print media. By appropriate manipulation of the compositions of each of two types of ink, it is possible to form a defined boundary free of Halo or Bleed artifacts, as taught for example in U.S. Pat. No. 5,198,023 (Stoffel--Cationic Dyes With Added Multi-Valent Cations To Reduce Bleed In Thermal Ink-Jet Inks) which is hereby incorporated by reference.
However, a typical subtractive color printing process requires not just two, but up to four types of ink (black, cyan, magenta, and yellow), which results in 4.times.3/2=6 possible pairings (4.times.3=12, if order is critical), or up to 6+4+1=11 combinations (12+24+24=60 combinations, if order is critical) if more than 2 types are applied to the same location. Thus, ink formulations that rely on composition to produce a desirable interaction between a particular pair of inks may produce an unintended (and unwanted) interaction with other combinations of inks. For example, if the interactions are based on the ionic polarity (+ or -) of polyelectrolytes with two types of ink being cationic (+) and two being anionic (-), four of the six possible pair types will have opposing charges (+- or -+) and a resultant well-defined interface and two will have the same charge (++ or --) with significant resultant Bleed or Halo. Careful optimization of print medium and surface coatings interacting with the inks will typically produce a substantial improvement to image quality; however, use of special purpose print media materially adds to the cost per page and is therefore not practical for many applications. Moreover, specially formulated inks and print media typically require modification of other printing variables such as nozzle temperature, firing energy, drying time and temperature, with the result that the reformulated materials may not be suitable for use with existing printers.
Providing a heater or increasing the power of an existing heater can increase the ink drying rate and decrease the mobility of the ink, thus lessening the propensity for Halo and Bleed effects; however, excessive heater power can cause dry cockle, paper browning and paper curl. Moreover, heaters are not practical in energy sensitive portable printers and in ink jet printers designed for office use the heater power is already set to a practical maximum in order to maximize throughput.